Sol-gel hybrid materials containing precious metals as catalysts for partial oxidation of hydrocarbons

ABSTRACT

The present invention relates to a process for preparing a composition containing gold and/or silver particles and an amorphous, organic/inorganic titanium/silicon mixed oxide, the compositions which can be prepared by this process and their use as catalysts for the selective oxidation of hydrocarbons.

TECHNICAL FIELD OF THE INVENTION

The present invention provides a process for preparing a compositioncontaining gold and/or silver particles and an amorpous titanium/siliconmixed oxide, compositions which can be prepared in accordance with thisprocess and their use as catalysts for the partial oxidation ofhydrocarbons.

BACKGROUND OF THE INVENTION

The sol-gel process is known [L. C. Klein, Ann. Rev. Mar. Sci., 15(1985) 227; S. J. Teichner, G. A. Nicolaon, M. A. Vicarini and G. E. E.Garses, Adv. Colloid Interface Sci., 5 (1976) 245]. However, thisprocess has not been used hitherto to prepare compositions which containgold and/or silver particles and an amorphous titanium/silicon mixedoxide, as a method for preparing catalysts for direct oxidation withmolecular oxygen as a reducing agent, because the suitability ofcompositions prepared therefrom for the catalytic oxidation ofhydrocarbons has not been disclosed.

SUMMARY OF THE INVENTION

Crystalline titanium silicalite catalysts are known.

U.S. Pat. No. 4,833,260 describes crystalline titanium silicalitecatalysts which enable the effective epoxidation of olefins with theoxidising agent hydrogen peroxide in the liquid phase. In silicalites, asmall proportion of the silicon in the lattice has been replaced bytitanium (U.S. Pat. No. 4,410,501).

On platinum metal-containing titanium silicalites, propene oxidationproceeds with small yields (about 1-2%) and propene oxide selectivitiesof 60-70% in the liquid phase due to in situ hydrogen peroxideproduction using a gas mixture consisting of molecular oxygen andmolecular hydrogen (JP-A 92/352771, WO 97/47386, WO 96/023 023).Hydrogenations which occur as a secondary reaction lead to large amountsof propane as a secondary product and the fact that this is a liquidphase reaction in which the epoxide being produced accumulates in theliquid phase means that this process is of little interest forindustrial use.

U.S. Pat. No. 5 623 090 describes a gas phase direct oxidation ofpropene to propene oxide with relatively low propene conversions low(0.5-1% propene conversion, with respect to a 10% strength propene feedconcentration) but with propene oxide selectivities of >90% with oxygenas the oxidising agent. This is a gold/titanium dioxide catalysed gasphase oxidation with molecular oxygen in the presence of hydrogen attemperatures of 40-70° C. The catalyst which is used is commerciallyavailable crystalline titanium dioxide with a very high proportion ofthe anatase modification (P25, Degussa; 70% anatase and 30% rutile),which is coated with nano-scale gold partides using adeposition-precipitation method. This process has the largedisadvantage, in addition to relatively low propene conversions, thatthe disclosed catalysts deactivate greatly with time. Typical half livesat atmospheric pressure and 50° C. are 30-150 minutes. Increasing thetemperature and/or pressure in order to raise the conversion shortensthe half lives even further.

In another embodiment, with the same reactant gases, catalysts are usedin which gold particles are applied to a support consisting of finelydispersed titanium centres on a silicon dioxide matrix (analogous to theshell variant [U.S. Pat. No. 3,923,843], a heterogeneous titanium andsilicon-containing material is used, which is prepared by impregnatingSiO₂ with titanium precursors in solution) (WO 9800415 A1; WO 9800414A1; EP 0 827 779 A1). All these catalysts, which are obtained frommaterials by impregnation of the purely inorganic silica surface withtitanium precursors in solution followed by coating with gold bydeposition-precipitation and subsequent calcination in an atmosphere ofair, exhibit relatively low propene conversion and deactivate rapidly(typical half lives are 10-50 h) and therefore cannot readily be used inindustrial scale plant.

WO-98/00413 discloses catalysts in which gold particles are applied toinorganic, microporous, silicates with a crystalline structure withdefined pore structures (e.g. TS-1, TS-2, Ti-zeolites such as Ti-beta,Ti-ZSM-48 or titanium-containing, mesoporous molecular sieves such ase.g. Ti-MCM-41 or Ti-HMS). Although all these purely inorganicgold/silicate or gold/zeolite catalysts exhibit good selectivitiesduring partial oxidation, the conversions of hydrocarbons, and inparticular the catalyst lifetimes, are inadequate for application in thechemical industry.

The methods described for preparing the catalysts are highlyunsatisfactory with respect to catalyst activity and lifetime.Industrial processes which use low activity catalysts require very largereactors. Low catalyst lifetimes restrict production output during theregeneration phases or require duplicated, cost-intensive productionroutes.

Thus the development of a process to prepare catalysts with whichexcellent selectivities and high activities can be achieved withindustrially relevant lifetimes is required.

Furthermore there is a requirement for a domain-free structure in thecatalysts.

There is also the object of reducing the disadvantages in the processaccording to the prior art.

Another object of the present invention is to provide a technologicallysimple catalytic gas phase process for the selective oxidation ofhydrocarbons with a gaseous oxidising agent on economically viable solidcatalysts which leads to high yields and low costs with very highselectivities and industrially relevant catalyst lifetimes.

These objects are achieved according to the invention by the provisionof a process for preparing a supported composition which contains goldand/or silver particles and an amorphous titanium/silicon mixed oxide,characterised in that the titanium/silicon mixed oxide is prepared by asol-gel process and that organic/inorganic sol-gel hybrid systems arepreferably prepared.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 illustrates the DRIFT spectra of an organic/inorganic hybridmaterial in accordance with Example 1 and a purely inorganic sol-gelmaterial produced from tetraethoxysilane and tetrabutoxysilane.

DETAILED DESCRIPTION OF THE INVENTION

The supported composition which can be prepared according to theinvention contains gold and/or silver on a support material. In thecatalytically-active state, gold and/or silver are mainly present aselemental metals (analysis by X-ray absorption spectroscopy). Smallproportions of gold and/or silver may also be present in a higheroxidation state. According to TEM images it has been ascertained thatthe majority of the gold and/or silver is present on the surface of thesupport material. These gold and/or silver clusters are present on ananometre scale. Supported compositions in which the gold particles havea diameter in the range 0.5 nm to 50 nm, preferably 2 to 15 nm and inparticular 2.1 to 10 nm are preferred. Silver particles have a diameterin the range 0.5 to 100 nm, preferably 0.5 to 40 nm and in particular0.5 to 20 nm.

The gold concentration should preferably be in the range 0.001 to 4 wt.%, preferably 0.001 to 2 wt. % and in particular 0.005 to 1.5 wt. % ofgold.

The silver concentration should preferably be in the range 0.005 to 20wt. %, preferably 0.01 to 15 wt. % and in particular 0.1 to 10 wt. % ofsilver.

Higher gold and/or silver concentrations than the ranges mentioned abovedo not produce any increase in catalytic activity. For economic reasonsthe noble metal concentration should be the minimum amount required toprovide the highest catalytic activity.

A titanium/silicon mixed oxide in the context of the invention isgenerally understood to be a silicon component which is chemicallycombined with a titanium component, e.g. titanium oxide or hydroxide,and optionally other foreign oxides (promoters). This amorphoustitanium/silicon mixed oxide is brought into contact with gold and/orsilver. The polarity of the surface of a catalyst according to theinvention may optionally be adjusted in a targeted manner, e.g. usingsilylating agents and/or by incorporating hydrophobic groups in thesupport matrix (e.g. alkyl and/or aryl groups or fluorine).

Production of the noble metal particles on thetitanium/silicon-containing mixed oxides is not restricted to onemethod. To generate gold and/or silver particles, a few examples ofmethods, such as deposition-precipitation as described in EP-B-0 709 360on p. 3, line 38 et seq., impregnation in solution, incipient wetness,colloid processes, sputtering, CVD and PVD, are mentioned. It is alsopossible to integrate precursor compounds of the noble metals directlyinto the sol-gel process. After drying and annealing the noblemetal-containing gels, nano-scale gold and/or silver particles are alsoobtained. Incipient wetness is understood to be the addition of asolution containing soluble gold and/or silver compounds to supportmaterials, wherein the volume of the solution on the support is smallerthan or equal to the pore volume of the support. Thus the supportremains dry on a macroscopic scale. Any solvent may be used, as asolvent for incipient wetness, in which the noble metal precursorcompounds are soluble, such as water, alcohols, ethers, esters,acetates, ketones, halogenated hydrocarbons, amines, etc.

Nano-scale gold particles produced by the incipient wetness andimpregnation methods are preferred. Nano-scale silver particles producedby the incipient wetness, deposition-precipitation and impregnationmethods are preferred.

Surprisingly, the generation of nano-scale gold particles from solublegold compounds, such as tetrachloroauric acid, e.g. by the incipientwetness method, may also take place in the presence of oligomeric orpolymeric auxiliary substances such as polyvinylpyrolidone, polyvinylalcohol, polypropylene glycol, polyacrylic acid, etc. or in the presenceof complex-forming components such as cyanides, acetylacetone,ethylacetoacetate, etc. Complex-forming additives such as cyanides, e.g.alkali metal or alkaline earth metal cyanides, are preferably used.

Compositions according to the invention may advantageously be furtheractivated, before and/or after being coated with a noble metal, bythermal treatment at 100-1000° C. in various atmospheres such as air,nitrogen, hydrogen, carbon monoxide, carbon dioxide. Thermal activationat 150-300° C. in oxygen-contaiming gases such as air, oroxygen/hydrogen or oxygen/rare gas mixtures or combinations thereof orunder inert gases at 150-1000° C., such as nitrogen and/or hydrogenand/or rare gases or combinations thereof, is preferred. Activation ofcompositions according to the invention is particularly preferablyperformed under inert gases in the temperature range 200-600° C.However, it may also be advantageous to anneal the support materialsaccording to the invention at temperatures in the range 200-1000° C. andthen to coat these with a noble metal. Thermally activated (annealed)compositions according to the invention frequently exhibit asignificantly higher catalytic activity and an extended lifetime whencompared with known catalysts.

The mixed oxides in the context of the invention contain between 0.1 and20 mol % of titanium, preferably between 0.5 and 10 mol %, in particularbetween 0.6 and 6 mol %, with respect to silicon oxide. The titanium ispresent in an oxidic form and is preferably chemically incorporated intoor bonded to the mixed oxide via Si—O—Ti bonds. Active catalysts of thistype contain hardly any Ti—O—Ti domains.

In addition to titanium, compositions according to the invention maycontain other foreign oxides, so called promoters, from group 5 of theperiodic system according to IUPAC (1985), such as vanadium, niobium andtantalum, preferably tantalum, from group 3, preferably yttrium, fromgroup 4, preferably zirconium, from group 8 preferably Fe, from group 15preferably antimony, from group 13 preferably aluminium, boron, thalliumand metals from group 14, preferably germanium.

For the most part these promoters are advantageously homogeneouslydistributed, i.e. there is very little domain production. The promotersincorporated, “M”, are generally present in the mixed oxide materials ina dispersed form and are advantageously bonded via element—O—Si bonds.The chemical composition of these materials may vary over wide ranges.The proportion of promoter element, with respect to silicon oxide, is inthe range 0-10 mol %, preferably 0-4 mol %. Obviously, several differentpromoters may also be used. The promoters are preferably used in theform of promoter precursor compounds which are soluble in the particularsolvent involved, such as promoter salts and/or promoter-organiccompounds and/or promoter-organic-inorganic compounds.

These promoters may increase both the catalytic activity of thecomposition and also the lifetime of the composition in catalyticoxidation reactions of hydrocarbons.

In principle, any crystal structure may be selected for the siliconcomponent, but the amorphous modification is preferred. In principle,any crystal structure may be selected for the titanium oxide, but theamorphous titanium dioxide modification is preferred. Thetitanium/silicon mixed oxide does not have to be present as a purecomponent, but may also be present as a complex material, e.g. incombination with other oxides (e.g. titanates). According to ourinformation, the titanium centres which are chemically bonded to silicaand/or inorganic silicates, are the catalytically active centres.

The titanium-containing mixed oxide materials are prepared by sol-gelprocesses. This takes place, for example, by mixing suitable, generallylow molecular weight compounds, in a solvent, after which the hydrolysisand condensation reaction is initiated by adding water and optionallycatalysts (e.g. acids, bases and/or organometallic compounds and/orelectrolytes). Basically, a person skilled in the art knows how toperform such sol-gel processes.

Suitable precursor compounds for silicon, titanium and promoter centresare advantageously corresponding low molecular weight inorganic mixedcompounds which are suitable for the sol-gel process or preferably acombination of corresponding inorganic and organic/inorganic mixedcompounds. Low molecular weight in the context of the invention meansmonomers or oligomers. Given sufficient solubility, polymeric precursorcompounds of silicon, titanium and promoters are also suitable.

The titanium/silicon mixed oxide is prepared by simultaneouspolymerisation of suitable Si and Ti precursors e.g. copolycondensationsto give amorphous Xerogels or Aerogels or the like (sol-gel process).This sol-gel process is based on the polycondensation of hydrolysed,colloidally dissolved metal component mixtures (sol) with the productionof an amorphous, three-dimensional network (gel). The followingschematic diagram is provided as further explanation:

acid/base acid/base sol hydrolysis condensation gel network

Hydrolysis is performed by initially introducing hydrolysable siliconand titanium precursors into a suitable solvent and then mixing withwater and optionally homogenising the mixture with a minimal quantity ofdissolution promoter. Since the hydrolysis of silicon precursorcompounds under normal conditions is slow, catalysts are required inorder to enable it to proceed rapidly and completely (J. Livage et al.,Chemistry of Advanced Materials: An Overview (eds: L. V. Interrante etal., VCH, New York, 1998, p. 389-448). The silanols produced condensewith the formation of siloxane compounds. Dissolved polysiloxanenetworks are produced in this way. Branching and transversecross-linking continues until the polymer is so large that thetransition to a gel takes place. The gel initially consists of a solidpolymeric network which is infiltrated by solvent. During a subsequentdrying procedure, the network shrinks with loss of the solvent, whereina polysiloxane Xerogel is produced. If the gel is dried undersupereritical conditions, the product produced is called an Aerogel (A.Baiker et al., Catal. Rev. Sci. Eng. 1995, 37, 515-556).

Preferred solvents for the sol-gel process are alcohols such asisopropanol, butanol, ethanol, methanol or ketones such as acetone, orethers or chlorinated hydrocarbons. Suitable starting materials are anysoluble silicon and titanium compounds of the general formula (I) knownto a person skilled in the art and which may be used as startingmaterials for the corresponding oxides or hydroxides,

[R_(x)M′(OR′)_(4-x)]  (I),

wherein

M′ is selected from silicon and titanium,

R and R′ are identical or different and are selected, independently,from the group C₁-C₁₂ alkyl, C₁-C₁₂ alkylene and C₆-C₁₂ aryl, whereinx=0, 1, 2, 3 and R′ may also be H. Preferably X=1,2 or 3. R′ and R canbe an alkyl(aryl)silane, e.g. Trimethylsilyl, too.

In the case of preferred organically modified silanes, one or morehydrolysable groups have been replaced by terminal and/or bridgedsaturated (e.g. CH₃, C₂H₅, C₃H₇, etc.) or by unsaturated (e.g. C₂H₃,C₆H₅) R group(s). Polyfunctional organo-silanes, e.g. silanols andalkoxides, may also be used. Silanes, whether organically modified ornot, are reacted in the presence of dihydric or polyhydric alcohols,such as 1,4-butanediol, to give organically modified polysiloxanes.Bridged R groups (alkylene groups) in the context of the invention arebridged structures such as chain-shaped, star-shaped (branched),cage-shaped or ring-shaped structural elements.

Mixed oxides with organic components are called organic-inorganic hybridmaterials. Organic-inorganic hybrid materials according to thisinvention have terminating or bridging organic groups in theTi—Si—network. This organic-inorganic hybrid materials are preferred.

Polysiloxanes, e.g. Polydimethylsiloxane (PDMS), optionally withfunctionalised terminal groups such as hydroxyl or alkoxy, and/ordiphenylsilanediol, may also be homogeneously incorporated into thenetwork structure for the purpose of adjusting the polarity of thesurface in a targeted manner.

The organic-inorganic silicon- and titanium-precursor compounds can beused in combination with purely inorganic network forming compounds astertraethoxysilane, tertamethoxysilane, etc. In spite of monomericalkoxides, the respective condensation products van be used, e.g.Si(OC₂H₅)₄. Furthermore oligomeric or polymeric systems, e.g.polydiethoxysiloxanes, can be used.

The modified silanes preferably used here clearly differ from theconventionally used purely inorganic network-producers, such asalkoxysilanes [Si(OR)₄] with four hydrolysable groups, which are usede.g. for preparing crystalline silicate structures with defined porestructures (WO-98/00413; TS 1, TS 2, Ti-MCM 41 and 48).

In contrast to catalysts according to the invention, a common feature ofall previously known catalysts is that gold particles have been appliedto purely inorganic support materials, i.e. that the solid structureconsists of purely inorganic silicon/oxygen and titanium/oxygen units.

Alkyl is understood to be any terminal and/or bridged linear or branchedalkyl group with 1 to 12 carbon atoms known to a person skilled in theart, such as methyl, ethyl, n-propyl, i-propyl, n-butyl, i-butyl,t-butyl, n-pentyl, i-pentyl, neo-pentyl, hexyl and other homologueswhich may, for their part, also be substituted. Suitable substituentsare halogen, nitro or also alkyl, hydroxide or alkoxy, and cycloalkyl oraryl, such as benzoyl, trimethylphenyl, ethylphenyl, chloromethyl,chloroethyl, and nitromethyl. Non-polar substituents are preferred, suchas methyl, ethyl, n-propyl, i-propyl, n-butyl, i-butyl, t-butyl andbenzoyl. Higher molecular weight and/or oligomeric organic/inorganicsilicon and titanium precursors are also suitable, such asgamma-glycidoxypropyltrimethyloxysilane,3,4-epoxycyclohexyl-ethyl-trimethoxysilane,1-(triethoxysilyl)-2-(diethoxymethylsilyl)ethane,tris-(gamma-trimethoxypropyl) isocyanurate, peralkylated cyclosiloxanessuch as hexamethylcyclotrisiloxane, octamethyltetrasiloxane ordecamethylpentasiloxane. Polyalkyl(aryl)siloxanes such aspolydirnethylsiloxane are also suitable.

Aryl is understood to be any mononuclear or polynuclear aryl group with6 to 12 carbon atoms known to a person skilled in the art, such asphenyl, napthyl or fluorenyl, which, for their part, may also besubstituted. Suitable substituents are halogen, nitro or also alkyl oralkoxy, as well as cycloalkyl or aryl substituents, such as bromophenyl,chlorophenyl, toluyl and nitrophenyl. Phenyl, fluorenyl, bromophenyl,chlorophenyl, toluyl and nitrophenyl are preferred.

Examples are the corresponding alkoxides, soluble salts andorganosilicon or organotitanium compounds.

Although any salts, such as halides, nitrates and hydroxides may beused, the alkoxides, e.g. butoxide, isopropoxide, propoxide and ethoxideof these elements are preferred.

Titanium derivatives such as tetraalkoxytitanates, with C₁-C₁₂ alkylgroups, such as iso-butyl, tert-butyl, n-butyl, i-propyl, propyl, ethyl,etc. or other organic titanium species such as titanyl acetylacetonate,dicyclopentadienyltitanium dihalide, titanium dihalogenodialkoxide, andtitanium halogenotrialkoxide, preferably in combination with alkyl groupcontaining titanium derivatives, are preferably used. Chlorine ispreferred as a halogen substituent. Mixed oxides of titanium and otherelements such as e.g. titanium triisopropoxide tri-n-butylstannic oxidemay also be used. The titanium precursor compounds may also be used inthe presence of complex-forming components such as e.g. acetylacetone orethylacetoacetate.

The organic/inorganic silicon and titanium precursor compounds may alsobe used in combination with inorganic network producers such astetraethoxysilane (Si(OC₂H₅)₄) and tetramethoxysilane (Si(OCH₃)₄). Thecondensation products may also be used instead of the monomericalkoxides. For example, Si(OC₂H₅)₄ condensates are commerciallyavailable. Furthermore, oligomeric or polymeric systems such aspoly(diethoxysiloxane) may also be used.

If small amounts of tetraalkyl orthotitanates are replaced bytrialkoxytitanium species, e.g. trialkoxymethyltitanium, the surfacepolarity may also be adjusted. In addition to monomeric alkoxides,equally effective polymeric systems, such as e.g.poly-(diethoxysiloxanethyl titanate), poly-(octyleneglycol titanate)etc., may be used.

Tetraalkyl orthosilicates such as tetramethyl orthosilicate and/ortetraethyl orthosilicate and trialkoxymethylsilane are preferably used.

Coprecipitates or co-gels of Si, Ti and optionally promoters, Si and Ti,Si and optionally promoters, Ti and optionally promoters, or Si andoptionally promoters may also be used as starting compounds in theprocess according to the invention.

In particular for an industrial scale application, processes based onwater glass (an aqueous sodium silicate solution is hydrolysed e.g.after ion-exchange in acids, or a process in which silica is transferredto an organic solvent and then condensed in this medium by acid, neutralor basic catalysts) also provide preferred starting materials in thecontext of the invention, so that the so-called water glasses are alsopreferred.

The solvents used in the process according to the invention when usingwater-sensitive precursor compounds (e.g. alkoxides) are polar organicsolvents such as alcohols, e.g. methanol, ethanol, isopropanol, butanol,preferably ethanol, isopropanol or methanol, or other polar organicsolvents known to a person skilled in the art which do not have adisadvantageous effect in the sol-gel process such as acetone,sulfolane, or similar solvents, preferably acetone. When using so-calledwater glasses, water and organic solvents which are miscible with water,such as alcohols, are used, preferably water.

The compositions according to the invention which contain gold and/orsilver particles and titanium and silicon-containing materials may beapproximately described in the dry state by the cmpirical generalformula (II) given below (groups on the surface formed aftermodification and optionally incompletely reacted groups are not takeninto account here):

SiO_(x)*Org*TiO_(y)*MO_(z)*E  (II)

SiO_(x) represents silicon oxide, Org represents the organicconstituents in the formula, preferably produced in a sol-gel processfrom the organic/inorganic precursors, M is a promoter, preferably Ta,Fe, Sb, V, Nb, Zr, Al, B, Tl, Y, Ge or combinations thereof, Erepresents gold and/or silver (noble metal) and x, y and z are thenumber of oxygen atoms needed for effective saturation of the valencesof Si, Ti and M.

The composition called (II) above can be varied over wide ranges.

The proportion of Org as a molar percentage, with respect to siliconoxide, may be between 0 and 300%. It is preferably between 10 and 150%,in particular between 30 and 120%. The proportion of titanium oxide,with respect to silicon oxide is between 0.1 and 10 mol %, preferablybetween 0.5 and 8.0%, in particular between 2.0 and 7.0%. The proportionof MO_(z), with respect to silicon oxide, is between 0 and 12 mol %. Theproportion of E, with respect to the noble metal-free composition, isbetween 0.001 and 8 wt. %. In the case of gold it is preferably between0.001 and 2 wt. %, in the case of silver it is preferably between 0.01and 15 wt. %.

Furthermore, the objects mentioned above are solved by a process forpreparing compositions according to the invention which contain goldand/or silver particles and titanium and silicon-containing materials.

The sequence of operational steps during sol-gel synthesis is notdefined. The generation of catalysts according to the invention may beachieved, for example, by simultaneous hydrolysis and/or condensation ofSi and Ti precursors, by reaction of organic/inorganic precursorcompounds with appropriate Ti compounds followed by the optionaladdition of the appropriate Si compounds or by simultaneous reaction oforganic/inorganic precursor compounds of appropriate titanium andsilicon compounds.

In a preferred embodiment, the organic/inorganic silicon precursorcompound preferred here is initially introduced into a solvent,hydrolysed, with the addition of a catalyst, using an excess of water,with respect to the amount theoretically required, then the titaniumcompound is added and further water, optionally along with a catalyst,is added. After the production of a gel, which may take place withinfrom a few minutes to a few days, depending on the composition, thecatalyst, the amount of water and the temperature, the gel is driedimmediately or after an ageing period of up to 30 days or longer. Inorder to complete the hydrolysis and condensation reactions, the moistand/or already dried gel may optionally be treated, once or severaltimes, with an excess of water or water vapour. Drying in air or aninert gas is preferably performed at between 50 and 250° C., inparticular between 100 and 180° C.

The hydrophobicity of the organic/inorganic hybrid materials accordingto the invention is determined decisively by the number and type ofterminal and bridging Si—C bonds. These have, as compared with otherorganic bonds such as e.g. Si—O—C bonds, the additional advantage thatthey are largely chemically inert, i.e. they are insensitive tohydrolysis and oxidation reactions.

The noble metals may be added in the form of precursor compounds, suchas salts or organic complexes or compounds, during the sol-gel process,or else applied after production of the gel in a known manner e.g. byprecipitation, impregnation in solution, incipient wetness, sputtering,colloids, CVD. Surface modification of the composition is optionallyperformed after this stage.

Surface modification may be performed either before or after coatingwith a noble metal. DE 199 18 431.1 describes a supported compositionwhich contains gold and/or silver particles, titanium oxide and asilicon-containing support, which is characterised in that thecomposition has groups at the surface selected from siliconalkyl,siliconaryl, fluorine-containing alkyl or fluorine-containing arylgroups, and their use as catalysts for the direct oxidation ofhydrocarbons. Organic/inorganic hybrid materials have not been disclosedas a support.

Modifications in the context of the invention are understood to be inparticular the application of groups selected from siliconalkyl,siliconaryl, fluorine-containing alkyl or fluorine-containing arylgroups to the surface of the supported composition, wherein the groupsmay be bonded to the functional groups (e.g. OH groups) at the surfacein a covalent or coordinate manner. However, any other surface treatmentis expressly included within the scope of the invention.

For industrial applications, processes based on waterglass (an aqueoussodium silicate solution is hydrolysed, e.g. after ion-exchange inacids) or processes in which silicic acid is transferred to an organicsolvent and is then condensed in this medium under acid, neutral orbasic catalysis, also provide suitable titanium/silicon mixed oxides.

In the process according to the invention, acids or bases are used ascatalysts for the sol-gel process. Suitable acids and bases are known toa person skilled in the art, from the sol-gel literature such as L. C.Klein, Ann. Rev. Mar. Sci., 15 (1985) 227; S. J. Teichner, G. A.Nicolaon, M. A. Vicarini and G. E. E. Garses, Adv. Colloid InterfaceSci., 5 (1976) 245. Inorganic, aqueous or non-aqueous mineral acids suchas hydrochloric acid, nitric acid, sulfuric acid, phosphoric acid,hydrofluoric acid or similar and organic acids such as p-toluylsulfonicacids, formic acid, acetic acid, propionic acid, may be mentioned inparticular. Hydrochloric acid, nitric acid and p-toluylsulfonic acid areparticularly preferred.

The amounts of starting compounds used can be varied over a wide range.Typical molar ratios of hydrolysable Si(Ti) species to water are in therange 0.5-32, preferably 0.5-10.

Suitable catalyst support materials such as e.g. pyrogenic silica,Aerosils and/or Cabosils may also be suspended or dispersed in thecolloidal silica sols. Additional condensable, multifunctional moleculessuch as e.g. monomeric or polymeric glycols, metal halides, cellulose,gelatines or similar compounds may also be used for targeted “materialdesign” purposes; like the hydrolysed alkoxymetallates, these polymersmay be incorporated homogeneously into the gel network. The addition ofihydrophobic, organic solvents to the sol phase (dispersed phase), e.g.monofunctional aliphatic alcohols with more than eight carbon atoms,causes the production of an emulsion (dispersed sol phase andhomogeneous emulsion liquid) and thus enables design of the material tobe customised to a further extent.

The process is performed at pressures in the range from atmosphericpressure to 10 bar, in particular at atmospheric pressure.

The process is performed at temperatures in the range 0-100° C., inparticular at 10-60° C.

Any reactors and reactor inserts which have been described in the priorart are suitable as reactors.

The wet gels obtained in the process (called aqua, hydro or alko-gels)are dried in a conventional manner, that is by reducing the pressureand/or by increasing the temperature. The wet gels are advantageouslycrushed to a powder before drying. If moulded items, not powders, areintended to be formned, the sol is transferred into appropriateshape-providing moulds prior to gelling and then gelled and dried.Conventional drying is often associated with shrinkage of the initiallyobtained gel structure due to evaporation of liquid from the pores. Inorder to exchange pore liquid for air while retaining the filigree,solid network (Aerogels), special methods of drying have to be used.“Supercritical drying” with carbon dioxide is the method used mostfrequently.

Preparation of the final, amorphous, noble metal-containing compositionconsisting of titanium/silicon mixed oxide and gold and/or silverparticles is not restricted in any way.

The noble metal may be added in the form of precursor compounds such assalts or organic complexes or compounds during the sol-gel process, orelse may be applied after preparation of the gel in a known manner, e.g.by impregnation, incipient wetness or precipitation. Surfacemodification of the composition may optionally follow this stage, whenthe surface OH groups are covered with organic groups. Said surfacemodification may also take place after preparation of the gel and beforeapplication of the noble metal.

Amorphous compositions according to the invention may contain smallproportions of crystalline structures.

Although the morphology and particle size of the mixed oxides may bevaried over a wide range, homogeneous, amorphous mixed oxides with highsurface areas of >20 m²/g, preferably >50 m²/g are particularlypreferred. The specific surface area is determined in a conventionalmanner using Brunauer, Emmet and Teller's method, J. Anorg. Chem. Soc.1938, 60, 309, the pore volume is determined by the centrifuge methodaccording to McDaniel, J. Colloid Interface Sci. 1980, 78, 31 and theparticle size is determined using Cornillaut's method, Appl. Opt. 1972,11, 265.

The sol-gel process offers the opportunity of preparing extremelyhomogeneous and almost completely amorphous titanium/silicon mixedoxides. With high titanium concentrations (>10 wt. %), domain productionoccurs due to the preferred Ti—O—Ti homocondensation, in whichoctahedral Ti coordination, known from pure TiO₂, prevails. With dilute‘TiO₂ in SiO₂’ systems (<10 wt. % of Ti), homogeneous, i.e. domain-freeTi distribution takes place wherein the fourfold coordination preferredby silicon is also taken on by the titanium. These centres are probablythe catalytically-active centres (site-isolated centres) used for theselective oxidation of hydro-carbons. In addition the sol-gel processaccording to the invention is very versatile because gels of almost allmetal, semi-metal or non-metallic oxides are known and many of these aresuitable for the production of Xerogels and Aerogels, so that thetargeted introduction of foreign oxides into the lattice of thetitanium/silicon mixed oxides is in principle possible.

We have found that the selectivity and in particular the activity duringcatalysis of the oxidation of hydrocarbons can be increased if thecatalytically active metal centres are incorporated in a defined porearchitecture. Secondary reactions can be suppressed in this way. Thustitanium/silicon mixed oxides which have been prepared by a homogeneouscopolycondensation process, after coating with a noble metal (goldand/or silver) are highly active, selective, oxidation catalysts. Inparticular after optional treatment of the surface, these types ofsystems exhibit excellent selectivities and industrially relevantcatalyst lifetimes of weeks and longer.

The optionally present promoters which are described are present for themost part in a homogeneous distribution, i.e. there is very littledomain production, thanks to the sol-gel process.

The chemical flexibility of the chemical composition (type of metal,concentration of metal) and targeted modification of the catalystactivity, selectivity and lifetime as a result of optionally performedsurface modification, associated with the inhibition ofdeactivating/blocking processes, characterise the product from theprocess according to the invention.

Optionally performed surface treatment consists of treating with organicsilylating reagents. The resulting compositions are excellent, highlyselective redox catalysts.

Suitable silylating reagents are any known silicon compounds which areable to react with the surface OH groups (in a covalent or coordinatemanner). For instance, organic silanes, organic silylamines, organicsilylamides and their derivatives, organic silazanes, organic siloxanesand other silylating agents and also combinations of silylating reagentsmay be used as silylating reagents. Partly fluorinated or perfluorinatedalkyl(aryl)silicon organic compounds are also understood to be expresslyincluded among silylating compounds.

Specific examples of organic silanes are chlorotrimethylsilane,dichlorodimethyl-silane, chlorobromodimethylsilane,nitrotrimethylsilane, chlorotrimethylsilane, iododimethyl-butylsilane,chlorodimethylphenylsilane, chlorodimethylsilane,dimethyl-n-propyl-chlorosilane, dimethylisopropylchlorosilane,t-butyldimethylchlorosilane, tripropyl-chlorosilane,dimethyloctylchlorosilane, tributylchlorosilane, trihexylchlorosilane,dimethylethylchlorosilane, dimethyloctadecylchlorosilane,n-butyldimethylchlorosilane, bromomethyldimethylchlorosilane,chloromethyldimethyl-chlorosilane, 3-chloropropyl-dimethylchlorosilane,dimethoxymethylchlorosilane, methylphenylchlorosilane,triethoxychlorosilane, dimethylphenylchlorosilane,methylphenylvinylchlorosilane, benzyldimethylchlorosilane,diphenylchlorosilane, diphenylmethylchlorosilane,diphenylvinylchlorosilane, tribenzylchlorosilane and3-cyanopropyldimethyl-chlorosilane.

Specific examples of organic silylamines are N-trimethylsilylimidazoles,N-t-butyldimethylsilylimidazole, N-dimethylethylsilylimidazole,N-dimethyl-n-propylsilylimidazole, N-dimethylisopropylsilylimidazole,N-trimethylsilyldimethylamine, N-trimethylsilyldiethylaamine,N-trimethylsilylpyrrole, N-trimethylsilylpyrrolidine,N-trimethylsilylpiperidine, pentafluorophenyldimethylsilylamine and1-cyanoethyl(diethylamino)dimethylsilane.

Specific examples of organic silylamides and their derivatives areN,O-bistrimethylsilylacetamide, N,O-bistrimethylsilyltrifluoroacetamide,N-trimethylsilylacetamide, N-methyl-N-trimethylsilylacetamide,N-methyl-N-trimethylsilyltrifluoroacetamide,N-methyl-N-trimethylsilylheptafluorobutyrarnide,N-(t-butyldimethylsilyl)-N-tri-fluoro-acetamide andN,O-bis(diethylhydrosilyl)trifluroacetamide.

Specific examples of organic silazanes are hexamethyldisilazane,heptamethyl-disilazane, 1,1,3,3-tetramethyldisilazane,1,3-bis(chloromethyl)tetramethyldisilazane, 1,3-divinyl-1,1,3,3-tetramethyldisilazane and 1,3-diphenyltetramethyldisilazane.Examples of other silylating reagents areN-methoxy-N,O-bistrimethyl-silyltrifluoroacetamide,N-methoxy-N,O-bistrimethylsilyl carbamate, N,O-bistrimethyl-silylsulfamate, trimethylsilyltrifluoromethane sulfonate andN,N′-bistrimethylsilylurea.

Preferred silylating reagents arc hexamethyldisiloxane,hexamethyldisilazane, chlorotrimcthylsilane,N-methyl-N-trimethylsilyl-2,2,2-trifluoroacetamide (MSTFA) andcombinations of these silylating reagents.

Compositions which can be prepared according to the invention may alsobe subjected to water treatment prior to silylation in order to increasethe number of surface silanol groups. Water treatment in this connectionmeans that the catalyst is brought into contact with liquid water or anaqueous saturated ammonium chloride solution and/or ammonium nitratesolution and/or is ion-exchanged with polyvalent cations, e.g. aqueoussolutions of Ca²⁺, Eu³⁺ prior to the silylating process step, e.g. thecatalyst is suspended in water and then dried (e.g. at 300° C.), or thecatalyst is treated with water vapour at >100° C., preferably at150-450° C., for 1-6 h. The catalyst is particularly preferably treatedwith water vapour at 200-450° C. for 2-5 h and then dried and surfacemodified.

Compositions obtainable in the process according to the invention may beused in any physical form at all for oxidation reactions, e.g. powders,milled powders, spherical particles, granules (e.g. produced byspray-drying or spray-granulating), pellets, extnidates, etc.

Compositions obtainable in the process according to the invention areextremely suitable for oxidising hydrocarbons in the gas phase in thepresence of gases which contain (atmospheric) oxygen and hydrogen oroxygen and carbon monoxide; this use is another object of the invention.

As a result of gas phase reactions of oxygen and hydrogen in thepresence of compositions obtainable by the process according to theinvention, epoxides are obtained selectively from olefins, ketones areobtained selectively from saturated secondary hydrocarbons and alcoholsare obtained selectively from saturated tertiary hydrocarbons. Thecatalyst lifetimes, depending on the reactants used, extend to manymonths or longer.

The relative molar ratio of hydrocarbon, oxygen, hydrogen and optionallya diluent gas may be varied over a wide range.

The molar amount of hydrocarbon used, with respect to the total numberof moles of hydrocarbon, oxygen, hydrogen and diluent gas, may be variedover a wide range. An excess of hydrocarbon, with respect to the oxygenused (on a molar basis) is preferably used. The hydrocarbon content istypically greater than 1 mol % and less than 80 mol %. Hydrocarboncontents in the range 5 to 60 mol % are preferred, in particular 10 to50 mol %.

The oxygen may be used in a wide variety of forms, e.g. molecularoxygen, air and nitrogen oxide. Molecular oxygen is preferred. The molarproportion of oxygen, with respect to the total number of moles ofhydrocarbon, oxygen, hydrogen and diluent gas, may be varied over a widerange. The oxygen is preferably used in a molar deficiency with respectto the hydrocarbon. 1-30 mol % of oxygen is preferably used, inparticular 5-25 mol % of oxygen.

In the absence of hydrogen, the supported compositions according to theinvention exhibit only very low activity and selectivity. Up to 180° C.,the productivity in the absence of hydrogen is low, at temperaturesabove 200° C. large amounts of carbon dioxide are produced in additionto partial oxidation products. Any known source of hydrogen may be usedsuch as e.g. pure hydrogen, synthesis gas or hydrogen from thedehydrogenation of hydrocarbons and alcohols. In another embodiment ofthe invention, the hydrogen may also be produced in situ in an upstreamreactor, e.g. by the dehydrogenation of propane or isobutane or alcoholssuch as e.g. isobutanol. The hydrogen may also be introduced into thereaction system as a complex-bonded species, e.g. a catalyst/hydrogencomplex. The molar proportion of hydrogen, with respect to the totalnumber of moles of hydrocarbon, oxygen, hydrogen and diluent gas, may bevaried over a wide range. Typical hydrogen concentrations are greaterthan 0.1 mol %, preferably 4-80 mol %, in particular 5-65 mol %.

In addition to the reactant gases which are required as essentialconstituents, a diluent gas, such as nitrogen, helium, argon, methane,carbon dioxide, carbon monoxide or similar gases which behave asfundamentally inert gases, may optionally be used. Mixtures of the inertcomponents described may also be used. The addition of inert componentsis beneficial with regard to the transport of the heat being releasedduring this exotherinic oxidation reaction and also from a safety pointof view.

If the process according to the invention is performed in the gas phase,gaseous diluent components such as, e.g. nitrogen, helium, argon,methane, and optionally water vapour and carbon dioxide may be used.Although water vapour and carbon dioxide are not completely inert, theyhave a positive effect at very small concentrations (<2 vol. %).

We have found that the selective oxidation reaction described above isvery sensitive to the structure of the catalyst. Given the presence ofnano-disperse gold and/or silver particles in the supported composition,an advantageous increase in productivity to give the selective oxidationproduct was observed.

Furthermore, problems related to the diffusion of reactants and productscan be minimised when using catalysts according to the invention bydeliberately adjusting the polarity of the matrix to the requirements ofthe catalytic reaction. In order to produce a low polarity for thepolymer while retaining sufficient mobility of the reactive centres,cocondensation agents with non-polar hydrocarbons have to be integratedinto the polymer. The polarity and swelling behaviour of the support canalso be advantageously modified by incorporating oxophilic elementsother than silicon, such as boron, aluminium, yttrium, tantalum,zirconium or titanium. The choice of these heteroatoms is restricted,according to the invention, to elements which have redox-stableoxidation states.

Basically, the process according to the invention may be applied to anyhydrocarbons. The expression hydrocarbon is understood to mean anunsaturated or saturated hydrocarbon such as olefins or alkanes, whichmay also contain heteroatoms such as N, O, P, S or halogens. The organiccomponent to be oxidised may be acyclic, monocyclic, bicyclic orpolycyclic and may be a monoolefin, diolefin or polyolefin. In the caseof organic components with two or more double bonds, the double bondsmay be conjugated or non-conjugated. Hydrocarbons are preferablyoxidised from which the oxidation products which are produced havepartial pressures which are sufficiently low for the product to becontinuously removed from the catalyst. Unsaturated and saturatedhydrocarbons with 2 to 20, preferably 2 to 10 carbon atoms, inparticular ethene, ethane, propene, propane, isobutane, isobutylene,1-butene, 2-butene, cis-2-butene, trans-2-butene, 1,3-butadiene,pentene, pentane, 1-hexene, 1-hexane, hexadienes, cyclohexene, benzeneare preferred.

The invention also provides for use of the compositions which can beobtained in the process according to the invention as catalysts in aliquid phase process for the selective oxidation of hydrocarbons toepoxides in the presence of organic hydroperoxides (R—OOH) or in thepresence of gases which contain oxygen and hydrogen or oxygen and carbonmonoxide.

Compositions according to the invention can be prepared on an industrialscale in an economically viable process which involves no chemicalengineering problems.

The characteristic properties of the present invention arc explained inmore detail by the catalyst preparations and catalytic test reactionsused in the following examples.

Clearly, it is understood that the invention is not restricted to thefollowing examples.

EXAMPLES

Instructions for testing the catalysts (test instructions)

A metal tubular reactor with an internal diameter of 10 mm and a lengthof 20 cm and which has been set to a constant temperature by means of anoil thermostat is used. The reactor is supplied with reactant gasesusing a set of four mass flow regulators (hydrocarbon, oxygen, hydrogen,nitrogen). For reaction, 500 mg of powdered catalyst are initiallyintroduced at 140° C. and at atmospheric pressure. The reactant gasesare introduced to the reactor from above. The standard catalyst loadingis 3 l/g of catalyst/h. Propene was selected as an example of a‘standard hydrocarbon’. To perform the oxidation reactions, a gas streamenriched with nitrogen was selected, always referred to as a standardgas composition in the following: N₂/H₂/O₂/C₃H₆=14/75/5/6%. The reactiongases are analysed quantitatively on a gas chromatograph. Gaschromatographic separation of the individual reaction products isperformed by a combined FID/TCD method, in which the gases flow throughthree capillary columns:

FID: HP innowax, 0.32 mm internal diameter, 60 m long, 0.25 μm layerthickness:

TCD: the following arc connected in series:

HP-PLOT Q, 0.32 mm internal diameter, 30 m long, 20 μm layer thickness

HP-PLOT molecular sieve 5 A, 0.32 mm internal diameter, 30 m long, 12 μmlayer thickness:

Example 1

This example describes the preparation of a catalyst consisting of asilicon and titanium-containing, organic/inorganic hybrid material whichwas coated with gold particles (0.1 wt. %) using incipient wetness. Theconcentration of non-hydrolysable organic components is 68 mol % andthat of titanium is 3.8 mol %, with respect to silicon.

1.9 g of a 0.1 N solution of p-toluenesulfonic acid in water were addedto 10.1 g of methyltrimethoxysilane (74.1 mmol) and 15 g of ethanol (AR)and the mixture was stirred for 2 hours. Then 1.46 g oftetrabutoxytitanium (4.3 mmol) were added slowly, the mixture wasstirred for a further 30 minutes, 7.1 g of tetraethoxysilane (34.1 mmol)were added, the mixture was stirred for 30 minutes, a mixture of 1.6 gof a 0.1 N solution of p-toluenesulfonic acid in water was added and themixture was then allowed to stand. The mixture reached gel-point after 3days. After an ageing period of 48 h, the gel was ground up in a mortarand dried for 8 h at 120° C. under air.

5.4 g of sol-gel material were impregnated with a solution consisting of540 mg of a 1% strength methanolic gold solution (HAuCl₄×3H₂O; Merck).which had been made up to 2.8 g with methanol, the macroscopically drymaterial was dried for 4 h at room temperature and then annealed for 2 hat 400° C. under an atmosphere of nitrogen.

In a test in accordance with the test instructions, a constant POselectivity of 95% was achieved. The maximum PO yield of 4% was achievedafter 2 h, and this dedined to 2.5% after 4 days.

Example 2

This example describes the preparation of a catalyst analogous toexample 1, but the gold-containing material dried at room temperaturewas annealed for 2 h at 400° C. under an atmosphere of hydrogen.

In a test in accordance with the test instructions, a constant POselectivity of 95% was achieved. The maximum PO yield of 4.1% wasachieved after 3 h, and this dedined to 2.8% after 4 days.

Example 3

This example describes the preparation of a catalyst consisting of asilicon and titanium-containing, organic/inorganic hybrid material whichwas coated with gold partides (0.5 wt. %) by thedeposition-precipitation method.

The sol-gel mixture is produced in the same way as in example 1.

2 g of support were initially introduced into 15 ml of methanol (Merck,AR), 20 mg of HAuCl₄×3H₂O (0.1 mmol, Merck) dissolved in 5 ml ofmethanol, were added thereto, the pH was adjusted to 8 with 0.5 ml of 1N Na₂CO₃ solution, the mixture was stirred for 30 min, 2 ml ofmonosodium citrate solution (32.1 g/l; pH=8) were added, the pH waschecked again and the mixture was stirred for 60 min. The solids wereisolated, washed 3 times with 20 ml of methanol each time, dried for 10h at 120° C. at atmospheric pressure, calcined for 5 h at 200° C. in airand then annealed for 2 h at 400° C. under nitrogen. The gold content ofthe gold/titanium/silicon catalyst was 0.48 wt. % (ICP analysis).

In a test in accordance with the test instructions, a constant POselectivity of 95% was achieved. The maximum PO yield of 2.5% wasachieved after 1 h, and this declined to 1.5% after 4 days.

Example 4

This example describes the preparation of a purely inorganic catalystsupport consisting of the oxides of silicon and titanium, which iscoated with gold particles by the precipitation method and thensurface-modified.

26 g of tetraethoxysilane (120 mmol, TEOS, Acros, 98%) were added to22.5 ml of i-propanol, thoroughly mixed and then 2.25 g of 0.1 N HClwere added thereto and the mixture was stirred for 2 h. 1.06 g oftetrabutoxytitanium (3.1 mmol, Acros, 98%) were added dropwise to thissolution and the mixture was stirred for 15 min. 23 ml of 2% strengthaqueous NH₃ solution were added to the homogeneous mixture. The mixturereached gel-point after about 5 min, was allowed to stand for 10 h andthen dried, initially for one hour at 120° C. at atmospheric pressure,then for about 20 h under vacuum (50 mbar) and calcined for 3 h at 300°C.

4 g of titanium-containing support were initially introduced into 35 mlof water, 70 mg of HAuCl₄ (0.178 mmol, Merck) in 5 ml of water wereadded thereto, the pH was adjusted to 8 with 1.1 ml of 2 N K₂CO₃, themixture was stirred for 30 min and 4 ml of monosodium citrate solutionwere then added, the pH was checked again and the mixture was stirredfor 120 min. The solids were isolated, washed three times with 40 ml ofwater each time, dried for 10 h at 120° C. at atmospheric pressure andcalcined for 3 h at 300° C. The gold content of thegold/titanium/silicone catalyst was 0.52 wt. % (ICP analysis).

2.5 g of substance and 0.5 g of 1,1,1,3,3,3-hexamethyldisilazane (3mmol, Merck) were initially introduced into dry hexane with stirring,stirred for 2 h at 60° C., the solids were filtered off, washed with 50ml of hexane and dried for 5 h at 120° C. under vacuum. Surfacemodification by silylation was then repeated.

In a test in accordance with the test instructions, a constant POselectivity of 95% was achieved. The maximum PO yield of 1.1% wasachieved after 1 h, and this declined to 0.8% after 4 days.

Example 5

This example describes the preparation of a catalyst analogous toexample 1, but 60 min after the addition of tetrabutoxytitanium 0.35 gof Ta(OEt)₅ (0.75 mmol, Chempur, 99.9%) were added to the homogeneousmixture, stirred for 15 min and the mixture was then gelled, worked up,coated with gold and annealed in the same way as in example 1.

In a test in accordance with the test instructions, a constant POselectivity of 95% was achieved. The maximum PO yield of 4.6% wasachieved after 4 h, and this declined to 4.0% after 4 days.

Example 6

This example describes the preparation of a catalyst analogous toexample 1, but 60 min after the addition of tetrabutoxytitanium 220 mgof Al(OC₄H₉)₃ (0.9 mmol, Chempur, 99.9%) were added to the homogeneousmixture, stirred for 15 min and the mixture was then gelled, worked up,coated with gold and annealed in the same way as in example 1.

In a test in accordance with the test instructions, a constant POselectivity of 95% was achieved. The maximum PO yield of 3% was achievedafter 2 h, and this declined to 2.0% after 4 days.

Example 7

Comparison example in accordance with EP-A1-827771

This example describes the preparation of a purely inorganic catalystsupport, consisting of the oxides of silicon and titanium, which wascoated with gold particles. The silicon and titanium-containing catalystsupport was obtained by impregnating silica with titanylacetylacetonate.

30 g of Aerosil 200 (pyrogeric silicon dioxide, Degussa, 200 m²/g) aresuspended in 250 ml of dry methanol, 0.98 g of titanyl acetylacetonate(3.9 mmol, Merck) are added thereto and the mixture is stirred for 2 hrat room temperature. The suspension is evaporated to dryness on a rotaryevaporator, the solid is then dried at 130° C. and calcined at 600° C.for 3 hr in a stream of air.

0.16 g of tetrachloroauric acid (0.4 mmol, Merck) is dissolved in 500 mlof distilled water, adjusted to a pH of 8.8 with 2N sodium hydroxidesolution, heated to 70° C., 10 g of the titanium-containing silicadescribed above is added thereto and the mixture is stirred for 1 hr.The solid is filtered off, washed with 30 ml of distilled water, driedfor 10 hr at 120° C. and calcined for 3 hr at 400° C. in air. Thecatalyst contains 0.45 wt. % of gold according to ICP analysis.

In a test in accordance with the test instructions, with a POselectivity of 92%, a propene conversion of 2.3% was achieved after 20min, the propene conversion was 1.5% after 100 min, the propeneconversion was 1.0% after 4 h and the propene conversion was 0.5% after10 h. Catalyst deactivation increased further with increasing time.

Example 8

Trans-2-butene is selected instead of propene as the unsaturatedhydrocarbon. For the partial oxidation of trans-2-butene, anorganic/inorganic hybrid catalyst consisting of the oxides of siliconand titanium, and which had been coated with gold particles, is used.The catalyst is prepared in the same way as described in example 1.

In a test in accordance with the test instructions, a constant2,3-epoxybutane selectivity of 91% was achieved. The maximum yield of 3%was achieved after 2 h, and this declined to 3.0% after 4 days.

Example 9

Cyclohexene is selected instead of propene as the unsaturatedhydrocarbon. For the partial oxidation of cyclohexene, anorganic/inorganic hybrid catalyst consisting of the oxides of siliconand titanium, which had been coated with gold particles, is used. Thecatalyst is prepared in the same way as described in example 1.Cyclohexene is taken into the gas phase with the assistance of anevaporator.

In a test in accordance with the test instructions, a constantcyclohexene oxide selectivity of 90% was achieved. The maximum yield of2.1% was achieved after 3 h, and this declined to 1.8% after 4 days.

Example 10

1,3-butadiene is selected instead of propene as the unsaturatedhydrocarbon. For the partial oxidation of 1,3-butadiene, anorganic/inorganic hybrid catalyst consisting of the oxides of siliconand titanium, which had been coated with gold particles, is used. Thecatalyst is prepared in the same way as described in example 1.

In a test in accordance with the test instructions, a constant buteneoxide selectivity of 82% was achieved. The maximum yield of 1% wasachieved after 4 h, and this declined to 0.7% after 4 days.

Example 11

Propane is used instead of propene, as a saturated hydrocarbon. For thepartial oxidation of propane, an organic/inorganic hybrid catalystconsisting of the oxides of silicon and titanium which had been coatedwith gold particles, is used. The catalyst is prepared in the same wayas described in example 1.

In a test in accordance with the test instructions, a constant acetoneselectivity of 80% was achieved. The maximum yield of 0.9% was achievedafter 4 h, and this declined to 0.7% after 4 days.

Characterising the Catalysts

Organic modification to the external and internal surfaces can bedemonstrated, for example, by so-called DRIFTS spectroscopy. DRIFTS(diffuse reflectance infra-red fourier transform spectroscopy) is awell-established vibration spectroscopic method for the structuralcharacterisation of functional groups and adsorbates on solid surfaces.Data on the principle of the method and some application examples fromthe field of heterogeneous catalysis may be found e.g. in the article byMestl, G., Knözinger, H., in the Handbook of Heterogeneous Catalysis,Vol. 2, p. 539 et seq. (VCH, Weinheim 1997), and the literature citedtherein.

To characterise the catalyst materials according to the invention withand without organic modification to the network, appropriate sampleswere stored for a few hours at 200° C. in a drying cabinet, transferredto an inert gas cell in the hot state and investigated spectroscopicallyby means of DRIFTS without further contact with air (to avoid there-adsorption of H₂O at the surface of the samples).

FIG. 1 shows the DRIFT spectra of an organic/inorganic hybrid material(in accordance with example 1) and a purely inorganic sol-gel materialproduced from tetraethoxysilane and tetrabutoxysilane). The clearlydetectable bands at about 3000 cm⁻¹ in the spectrum of theorganic/inorganic hybrid material are assigned to the homogeneousincorporation of hydrocarbons (CH₃ groups). The purely inorganicmaterial also contains hydrocarbon groups (traces) which possibly arisefrom the sol-gel process used to prepare the support (these hydrocarbongroups are obviously not fully thermally degraded by treating thematerial at a temperature of 200° C.).

What is claimed is:
 1. A process for preparing a noble metal-containingcatalyst comprising: (I) preparing a titanium/silicon mixed oxide by aprocess comprising: (a) providing a sol-gel material comprising (i) atitanium component and (ii) a silicon component and, optionally, (iii)an oxide promoter which is not titanium oxide; (b) optionally, allowingthe sol-gel material to age; (c) drying the sol-gel material to form atitanium/silicon mixed oxide; and (II) adding gold and/or silver to thetitanium/silicon mixed oxide to form a noble metal-containing catalyst.2. The process according to claim 1, wherein the titanium/silicon mixedoxide is an organic/inorganic hybrid.
 3. The process according to claim1, wherein the titanium/silicon mixed oxide comprises terminal and/orbridging organic groups.
 4. The process of claim 1, wherein the dryingis performed in air or inert gas at temperatures in the range of betweenabout 50° C. and 250° C.
 5. The process according to claim 1, whereinthe titanium component is titanium oxide or titanium hydroxide.
 6. Theprocess according to claim 1, wherein the optional oxide promoter isvanadium, niobium, tantalum, yttrium, zirconium, antimony, aluminum,boron, thallium or germanium.
 7. The process according to claim 1,wherein the titanium component is present is an amount between 0.1 and20 mol %, based on the amount of the silicon component and, if present,the oxide promoter.
 8. The process according to claim 1, wherein theoptional oxide promoter is present is an amount in the range from 0 mol% up to about 10 mol %, based on the amount of the silicon component. 9.The process according to claim 1, wherein the titanium component isamorphous.
 10. The process according to claim 1, wherein the siliconcomponent is amorphous.
 11. The process according to claim 1, whereinthe surface of the noble metal-containing catalyst comprisesorganosilicon and/or fluorine containing organic compounds.
 12. Theprocess according to claim 1, wherein the gold and/or silver is presenton the surface of the noble metal-containing catalyst in the form ofclusters.
 13. The process according to claim 1, wherein the gold has adiameter in the range of from about 0.5 to about 50 nm.
 14. The processaccording to claim 1, wherein the silver has a diameter in the range offrom about 0.5 to about 100 nm.
 15. The process according to claim 1,wherein the concentration of gold added to the titahium/silicon mixedoxide is in the range of from about 0.001 to 4 wt. %, based on the totalweight of the noble metal-containing catalyst.
 16. The process accordingto claim 1, wherein the concentration of silver added to thetitanium/silicon mixed oxide is in the range of from about 0.005 to 20wt. %, based on the total weight of the noble metal-containing catalyst.17. The process according to claim 1, wherein the volume of the goldand/or silver added to the titanium/silicon mixed oxide is equal to orsmaller than the pore volume of the titanium/silicon mixed oxide. 18.The process according to claim 1, wherein the surface polarity of thenoble metal-containing catalyst is adjustable.
 19. The process accordingto claim 1, wherein the addition of gold and/or silver particles is bydeposition-precipitation, impregnation, incipient wetness, colloidprocessing, sputtering, CVD, PVD or integration.
 20. The processaccording to claim 1, wherein the titanium/silicon mixed oxide isthermally activated.
 21. The process according to claim 20, wherein thegold and/or silver is added before or after thermal activation.
 22. Theprocess according to claim 20, wherein thermal activation is attemperatures in the range of from about 100° C. to about 1000° C. 23.The process according to claim 20, wherein the thermal activation occursin air, nitrogen, hydrogen, carbon monoxide, or carbon dioxide attemperatures in the range of from about from about 200° C. to about 800°C.
 24. The process according to claim 1, wherein the noblemetalcontaining catalyst is thermally activated.
 25. The processaccording to claim 24, wherein thermal activation is at temperatures inthe range of from about 100° C. to about 1000° C.
 26. The processaccording to claim 24, wherein thermal activation occurs in air,nitrogen, hydrogen, carbon monoxide, or carbon dioxide at temperaturesin the range of from about from about 200° C. to about 800° C.
 27. Theprocess according to claim 1, wherein the sol-gel material is formed by:a) introducing the silicon component, water and, optionally, a catalyst,into a solvent to form a silicon mixture; b) adding the titaniumcomponent, water, and, optionally, the oxide promoter, and, optionally acatalyst, into the silicon mixture to form a titanium/silicon mixture;c) allowing the titanium/silicon material to gel to form a solgelmaterial.
 28. The process according to claim 27, wherein the catalyst isan acid, a base, an organometallic compound and/or an electrolyte. 29.The process according to claim 28, wherein the catalyst is hydrochloricacid, nitric acid, sulfuric acid, phosphoric acid, hydrofluoric acid,p-toluylsulfonic acid, formic acid, acetic acid, or propionic acid. 30.The process according to claim 27, wherein the solvent is methanol,ethanol, isopropanol, butanol, acetone or sulfolane.
 31. The processaccording to claim 27, wherein step a) is a process based on waterglass.
 32. The process according to claim 31, wherein the solvent in thewater glass process is water and an organic solvent which is misciblewith water.
 33. A process for the oxidation of hydrocarbons in thepresence of molecular oxygen and a reducing agent wherein the oxidationof hydrocarbons is conducted in the presence of the noblemetal-containing catalyst of claim
 1. 34. The process according to claim33, wherein the hydrocarbon is propene.
 35. The process according toclaim 34, where propene is oxidized to propene oxide.
 36. A noblemetal-containing catalyst according to claim 1, wherein the noblemetal-containing catalyst is used for the oxidation of hydrocarbons inthe presence of molecular oxygen and a reducing agent.